Automated data storage libraries provide a means for storing large quantities of data in data storage media that are not permanently mounted on data storage drives, and that are stored in a readily available form on storage shelves. One or more robot accessors retrieve selected data storage media from storage shelves and provide them to data storage drives. Typically, data stored on data storage media of an automated data storage library, once requested, is needed quickly. Positional accuracy is required so that the robot accessor is able to access a desired storage shelf or a desired data storage drive, typically by direct “X” horizontal movement and “Y” vertical movement. The X and Y positioning are typically based on a reference or origin location.
Often, a library may comprise one or more calibration points in the library, so that the robot accessor can go to the calibration point and precisely measure its location to establish the parameters, including the X and Y location, needed to correctly access the remainder of the library.
Positional errors or uncertainty may develop through a number of means. One example is a library power off. The position of the robot accessor is not be monitored during the power off, and the robot accessor may have been moved. Another example is if a door of the library is opened and the robot accessor moved. Some other causes of positional errors may comprise belt or chain slippage or stretching, motor tachometer errors, control system errors, jumping a gear, a component may become misaligned or wear, etc.
Hence, a common practice involves having a home position for the robot accessor which is used as the reference or origin location for the X and Y positioning and for other features in the library. The home position may comprise a special home station at a position of the library, for example, at one end of the rails on which the robot accessor is moved. In a loss or uncertainty of position by the robot accessor, a “rezero” operation is conducted in which the robot accessor is moved toward the home position reference location. When at the home position, a home sensor indicates the location of the robot accessor, which then becomes known, and calibration or other operations can then be conducted. If its position is lost, the robot accessor is typically moved at a slow speed toward the home position for the rezero operation, since the actual position is unknown with any certainty, and the robot accessor may instead be actually moving to a crash stop, etc., and it is desirable to avoid a collision.
As automated data storage libraries become larger, the potential distance between the home position and the robot accessor becomes longer, and this increases the time it takes to perform the rezero operation.